We derived that the electric field due to an infinite sheet with charge density $\sigma$ was as follows: $$\mathbf{E}(z) = \begin{cases} \dfrac{\sigma}{2\varepsilon_0}\hat{k} & \mbox{if} & \mbox{ z > 0} \cr \dfrac{-\sigma}{2\varepsilon_0}\hat{k} & \mbox{if} & \mbox{ z < 0}\end{cases}$$ What does that tell you about the difference in the field when we cross the sheet, $\mathbf{E}(+z)-\mathbf{E}(-z)$? 1. it's zero 2. it's $\frac{\sigma}{\varepsilon_0}$ 3. it's -$\frac{\sigma}{\varepsilon_0}$ 4. it's +$\frac{\sigma}{\varepsilon_0} \hat{k}$ 5. it's -$\frac{\sigma}{\varepsilon_0} \hat{k}$ Note: * CORRECT ANSWER: D * Makes for a good discussion about cross one direction versus the other

## Announcements * Homework 2 solutions posted * Exam 1 is coming up! October 5th (More details next week!)

For me, the second homework was ... 1. fairly straight-forward; lower difficulty than I expected. 2. challenging, but at the level of difficulty I expected 3. a bit more difficult than I expected, but still manageable 4. much more difficult than I expected.

I spent ... hours on the second homework. 1. 1-4 2. 5-6 3. 7-8 4. 9-10 5. More than 10

## Electric Potential <img src="./images/lightning.jpg" align="center" style="width: 600px";/>

Which of the following two fields has zero curl? | I | II | |:-:|:-:| | <img src ="./images/cq_left_field.png" align="center" style="width: 400px";/> | <img src ="./images/cq_right_field.png" align="center" style="width: 400px";/> | 1. Both do. 2. Only I is zero 3. Only II is zero 4. Neither is zero 5. ??? Note: * CORRECT ANSWER: C * Think about paddle wheel * Fall 2016: 9 0 [89] 3 0

What is the curl of the vector field, $\mathbf{v}= c\hat{\phi}$, in the region shown below? <img src="./images/c_phi_field.png" align="right" style="width: 300px";/> 1. non-zero everywhere 2. zero at some points, non-zero at others 3. zero curl everywhere Note: * CORRECT ANSWER: A

What is the curl of this vector field, in the red region shown below? <img src="./images/curl_red_box.png" align="center" style="width: 400px";/> 1. non-zero everywhere in the box 2. non-zero at a limited set of points 3. zero curl everywhere shown 4. we need a formula to decide Note: * CORRECT ANSWER: D * I think it's D because it depends on how the field drops off, which we haven't indicated. If it's drops off like 1/r, then it has no curl.

What is the curl of this vector field, $\mathbf{v} = \dfrac{c}{s}\hat{\phi}$, in the red region shown below? <img src="./images/curl_red_box.png" align="center" style="width: 400px";/> 1. non-zero everywhere in the box 2. non-zero at a limited set of points 3. zero curl everywhere shown

Is it mathematically ok to do this? $$\mathbf{E} = \dfrac{1}{4\pi\varepsilon_0}\int_V\rho(\mathbf{r}')d\tau'\left(-\nabla\dfrac{1}{\mathfrak{R}}\right)$$ $$\longrightarrow \mathbf{E} =-\nabla\left( \dfrac{1}{4\pi\varepsilon_0}\int_V\rho(\mathbf{r}')d\tau'\dfrac{1}{\mathfrak{R}}\right)$$ 1. Yes 2. No 3. ???

If $\nabla \times \mathbf{E} = 0$, then $\oint_C \mathbf{E} \cdot d\mathbf{l} =$ 1. 0 2. something finite 3. $\infty$ 4. Can't tell without knowing $C$

Can superposition be applied to electric potential, $V$? $$V_{tot} \stackrel{?}{=} \sum_i V_i = V_1 +V_2 + V_3 + \dots$$ 1. Yes 2. No 3. Sometimes Note: As long as the zero potential is the same for all measurements.